High speed data service is supported in 1X EV-DV (“Physical Layer Standard for cdma2000 Spread Spectrum Systems, Release C) or HSDPA (“High Speed Downlink Packet Access: Physical Layer Aspects,” 3GPP TR 25.858) by using high level modulations, such as 16-QAM, and by using multiple channelization codes with smaller spreading factors. Reception of 16-QAM modulation symbols using a conventional RAKE receiver is very vulnerable in a multipath (dispersive) environment due to inter-code and inter-symbol interferences caused by diminished orthogonality among channelization codes. These structured interferences severely limit system peak data rate/throughput. High-speed data service is, however, considered to have most application in an urban area, where multipath channel is common. Therefore, a receiver with capabilities of mitigating inter-code and inter-symbol interferences is needed. For this purpose, such a receiver is proposed in copending patent application Ser. No. 60/360,382, assigned to the same assignee as the present application.
Hybrid ARQ (H-ARQ) is an implicit link adaptation technique. In adaptive modulation and coding (AMC), explicit carrier to interference (C/I) measurements or similar measurements are used to set the modulation and coding format. In H-ARQ, link layer acknowledgements are used for re-transmission decisions. There are many schemes for implementing H-ARQ, such as, Chase combining, Rate compatible Punctured Turbo codes and Incremental Redundancy. Incremental Redundancy or H-ARQ-type-II is another implementation of the H-ARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt.
H-ARQ retransmission is used in 1X EV-DV/HSDPA if some bits in a frame are not received correctly at a mobile unit. The retransmitted frame can be an exact copy of the previous frame (Chase Combining), or a subset of the previous frame (partial Chase Combining), or a different copy with different bits (Incremental Redundancy). However, all these retransmitted frames stem from the same systematic bits, and can thus be predicted at the mobile unit. Using this extra information, an enhanced decision feedback equalization scheme can be applied during the reception of H-ARQ retransmitted frames for further performance improvements.